Many dragons choose one area of their cage to potty in but even then they sometimes will potty elsewhere. Some people soak their dragons and have them poo in the bath. It is a matter of observing where your dragon prefers to go and providing items to make clean up easier. For instance, if you have tiles and notice that your dragon prefers to potty on the front center tile you can place a paper towel on that tile so he pottys on the paper towel and it doesn't run in between tiles.
When Mordecai was younger he would surf the glass in his tank and we new he wanted out but every time we took him out he would poo on us. Well it only took a few times and we realized he wanted out to go to the bathroom. So know every day about the same time we take him out put him in a tote with paper towels on the bottom and in a few minutes he goes potty and is good the rest of the day. He does not go potty in his Viv. It has been so nice. Just be consistent on what you decide to do and your dragon will get it. They are so smart. Good luck.
It takes patience and works better with older dragons. A lot of them respond to routine. I've found by taking them out at the same time every day for breakfast routine in an area with a linoleum floor that is easy to clean, they get used to pooping there. It takes a while to build the routine but it works pretty well once it's established.
I found with my troup of 5 beardies , that after I take them outdoors and sprinkle the hose sprinkler on them, they consistently go potty outside on the lawn.
I do this after their morning feeding of salad and roaches.
Then they spend a couple hours out in the sun in their non soiled reptarium. Saves a lot of time not cleaning up poo